Recently, it has become popular to process and manage the sound and video in a digital format in appliances such as acoustic appliances and video appliances. Such trends in digital encoding of sound and video in appliances such as acoustic appliances are extending to the field of radio broadcasting. For example, in the United States, a digital radio broadcast system called IBOC (In Band On Channel) is proposed and made available by iBiquity Digital Corp.
Meanwhile, conventional analog radio broadcast broadcasts via carrier (Hereinafter, “analog carrier”.) that has frequency distribution inside the frequency band (Hereinafter, “channel” or “frequency channel”.) assigned to individual broadcast stations. Actually, in order to avoid the interference between-analog carriers of adjacent channels, only the center portion of the assigned band is used for the transmission of the analog carrier, and other portions are not used. It is noted that “digital radio broadcast” in this application means “IBOC digital radio broadcast”.
IBOC is a type of a digital radio broadcast that uses frequency channel assigned to the conventional analog radio broadcast. In IBOC standard, a plurality of signal formats are defined, such as a hybrid format in which the digital radio broadcast signal is multiplexed onto the conventional analog radio broadcast signal, and an all digital format consisted of only digital signals, and it is designed to gradually transfer from conventional analog radio broadcast to all digital radio broadcast that has many functions and is high in quality. In the IBOC, digital broadcast signals are transmitted with Orthogonal Frequency Division Multiplexing (OFDM) that uses many carriers (subcarriers).
Also, in the IBOC standard, a signal format called a hybrid format is defined, which is used in the transition period from analog broadcast to all digital broadcast. In the hybrid format, it allocates the subcarriers of digital broadcast in the frequency portion that is adjacent to the center portion of the channel band which the analog carrier uses and that was not conventionally used. (Hereinafter, “sideband”.) In other words, in accordance with the hybrid format of the IBOC, the band allocated as the frequency channel for the conventional analog radio broadcast is utilized effectively, and the analog radio broadcast and the digital radio broadcast are simultaneously transmitted using a same channel.
For example, Japanese Patent Provisional Publication No. JP2004-349805 A1 (Hereinafter, “the Reference Document”.) discloses an IBOC broadcast receiver that is capable of receiving such IBOC digital radio broadcast. First, the IBOC broadcast receiver disclosed in the Reference Document filters the received signal to pass a broad frequency band that includes a center portion in which the analog carrier of the selected frequency channel is located and the portion (sideband) in which the adjacent subcarriers are located, and the subcarrier as well as the analog carrier is decoded. Then, if an IBOC signal (an identification information showing that it is a digital radio broadcast) is acquired as a result of the decoding, since the digital radio broadcast is transmitted in the selected frequency channel, the setting of the filtering is maintained to pass a broad range of band.
On the other hand, if an IBOC signal was not acquired, then only an analog radio broadcast is transmitted in the selected frequency channel, and no valid information is included in the sideband. In addition, not only the sideband does not include a valid information, it easily suffers from disturbances due to the adjacent band (It is the noise due to the adjacent frequency channel band. Hereinafter, it is called “adjacent disturbance”.), and it becomes the cause of the lowering of the carrier to noise ratio (CN ratio) of the selected frequency channel. Therefore, in case the IBOC signal was not acquired, then the signal of the selected frequency channel is filtered to pass the bandwidth of the analog carrier. Thereby, the sideband that is unneeded and easily affected by the adjacent disturbance is cut off, and the CN ratio for the analog radio broadcast is improved.
That is, the IBOC broadcast receiver disclosed in the above Reference Document is arranged to improve the CN ratio of the analog radio broadcast so that the band to pass the filter is enabled to be switched according to the existence of the IBOC signal, and the sideband is cut off to improve the CN ratio only if it is determined that the selected channel includes only an analog radio broadcast.
In the above Reference Document, in order to set the band used in the filtering process, as described above, it performs a decoding process to the selected frequency channel, and determines whether a digital radio broadcast is carried out or not in the frequency channel based on the result of the decoding process. However, the decoding process to decode the digital radio broadcast is a process that requires time. Therefore, in such an IBOC broadcast receiver, there is a problem in requiring time to set a filtering band.